1. Field of the Invention
The invention relates to a phosphor and a device using the same. More particularly, the invention relates to a phosphor having a magnetoplumbite-type crystal structure by which received light can be converted to lower energy (longer wavelength) light and relates to a device using such a phosphor. The phosphor of the invention can suitably be used for gas discharge devices such as fluorescent lamps and display devices such as plasma display panels (PDPs).
2. Description of the Related Art
Magnetoplumbite-type crystals such as aluminates are used as a typical phosphor along with crystals having a β-alumina-type structure. In order to form green phosphors of such crystals, for example, Tb is very often added to an appropriate phosphor host. Additionally, Ce3+ is often used as a sensitizing element for enhancing the emission from Tb3+. For example, CeMgAl11O19:Tb is such a phosphor with Ce3+ and reported in J. Luminescence, 9 (1974), pp. 415-419 or Philips Technical Review, 37 (1977) pp. 221-233.
In such a phosphor, the energy level of the Ce-emitted light is substantially equal to the fd-transition energy of Tb; therefore, the energy transition from Ce to Tb occurs with high efficiency. The excited Tb produces visible light emission based on the 5DJ→7FJ′ transition. More specifically, 5D4→7F5 transition produces a main peak of green emission at a wavelength of about 540 nm, 5D4→7F6 produces a sub-peak of blue emission at a wavelength of about 480 nm, 5D4→7F4 produces a sub-peak of yellow emission at a wavelength of about 580 nm, and 5D4→7F3 produces a sub-peak of red emission at a wavelength of about 600 nm.
The CIE color coordinate of the light emitted from this phosphor is approximately (0.31, 0.61). In this color coordinate, a y component represents a green component. Concerning the luminescence from the Tb-containing material such as CeMgAl11O19:Tb, a y value is at least 0.1 lower than that of the color coordinate of ZnSi2O4:Mn (0.21, 0.72) or BaMgAl14O23:Mn (0.15, 0.73); thus, such green color purity is considerably low.
Methods for improving the color purity of the Tb-containing phosphor are disclosed in Japanese Unexamined Patent Publication Nos. HEI 5(1993)-86366 and 2001-139942. The methods disclosed in these publications include adding Tb and Mn simultaneously to a magnetoplumbite-type crystal so that the color purity can be improved as compared to the material using only Tb.
The former of these publications discloses a phosphor of a formula (Ce1−xTbx)(Mg1−a−bZnaMnb)Al2zO2.5+3z, wherein 0<x≦0.6, 0<a+b<1, 4.5≦z≦15. This publication suggests that addition of Zn can increase the brightness of the phosphor containing Mn and Tb.
As an extension of the above known technology, the present inventors found that La3+ can produce light emission equal to Ce3+ when excited by vacuum ultraviolet light. Based on this finding, a phosphor of a formula (La1−xTbx)y(Mg1−a−bMnaZnb)AlzO1.5(y+Z)+1, wherein 0≦x≦0.5, 0.8≦y≦1.2, 0<a+b≦1, 8≦Z≦30 was invented (Japanese Unexamined Patent Publication No. 2003-342566). This phosphor is characterized by having higher brightness, when excited by vacuum ultraviolet, than the Ce-containing phosphor typically disclosed in Japanese Unexamined Patent Publication No. HEI 5(1993)-86366.
In order to be suitable for PDPs, mercury-free fluorescent lamps and the like, phosphors should further have improved properties as follows:
(1) They should have higher brightness.
(2) For display applications, their phosphorescence (afterglow) component should be as small as possible when they are excited by a 147 nm Xe atomic beam. As used herein, the term “phosphorescence” refers to a luminescence phenomenon remaining after the lapse of tens of milliseconds from the stop of excitation of phosphors. The use of a high-phosphorescence phosphor in display devices can create a problem that an image residue can remain even after displaying an image is stopped. On the other hand, when 172 nm light is used as an excitation source, phosphorescence can be reduced with known phosphors to a practically acceptable level.
Besides the above green phosphors, LaMgAl11O19:Eu2+ is known as a blue phosphor. This phosphor also produce s phosphorescence by vacuum ultraviolet excitation.
Phosphorescence is a weak luminescence phenomenon that occurs when excited electrons, holes and others are accumulated in traps such as defects and emitted by a thermal excitation process to reach a luminescent center. For example, it is detailed in Phosphor Handbook (edited by Phosphor Research Society, Ohmsha Ltd.).
Such a situation that excited carriers are trapped in defects or the like can cause not only phosphorescence but also brightness reduction. A radical solution to such phosphorescence lies in reducing traps such as defects. However, such a solution requires a drastic improvement in manufacturing technology and thus long time for research and development and high cost of development.